Polycyclic Aromatic Hydrocarbon Molecule with Ultra-low LUMO/HOMO Energy Levels★

doi:10.6023/A23040142

Abstract

Organic small molecules of low LUMO/HOMO (lowest unoccupied molecular orbital/highest occupied molecular orbital) energy levels are in shortage in terms of both variety and quantity. Their design and synthesis have important scientific and application value. The traditional strategy for designing organic small molecules of ultra-low LUMO/HOMO energy levels is to introduce multiple cyano groups into the molecules. In this work, a polycyclic aromatic hydrocarbon molecule containing four boron and nitrogen coordination bonds and two imide groups is designed and synthesized, without involving any cyano groups. The cyclic voltammetry examination with 0.1 mol•L⁻¹ tetrabutylammonium hexafluorophosphate solution in dichloromethane as the electrolyte solution and ferrocene as the internal standard indicates that the molecule has a low LUMO and HOMO energy level of －4.77 eV and －6.39 eV, respectively. These values are the lowest for the reported fused-ring small molecules with boron and nitrogen coordination bonds and comparable to those of the reported organic small molecules with cyano groups. The density functional theory calculation results at the B3LYP/6-31G(d,p) theoretical level show that the molecule has a curved configuration and its conjugate skeleton has a dihedral angle of 23.6°. Both LUMO and HOMO are uniformly delocalized on the linear benzoid skeleton. It shows obvious near-infrared absorption in both solution and thin film state, with maximum absorption at 768 nm in film. The molecule can be used as a p-type dopant. After its blend doping, the electrical conductivity of the film of a typical p-type polymer P3HT is improved by 3 orders of magnitude. The doping behavior is also confirmed by UV-Vis absorption spectroscopy and electron paramagnetic resonance spectroscopy. This work develops a new strategy to achieve ultra-low LUMO energy level for organic small molecules without using cyano groups.

Key words: energy level, boron nitrogen coordination bond, imide, p-doping, fused-ring molecule

Cite this article

Rui Liu, Bin Meng, Junli Hu, Jun Liu. Polycyclic Aromatic Hydrocarbon Molecule with Ultra-low LUMO/HOMO Energy Levels^★[J]. Acta Chimica Sinica, 2023, 81(10): 1295-1300.

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Fig. & Tab. 7

Figure 1. (a) The representative organic small molecules with LUMO energy levels lower than －4.60 eV reported in the literature. They all contain multiple cyano groups. Their LUMO levels are indicated. R is alkyl chain. (b) The organic small molecules based on boron and nitrogen coordination bonds and imide reported in this work. Its LUMO energy level is also indicated. R is isooctyl group

Figure 2. Synthetic routes of 4BN2I. Reagents and conditions: (I) CuCN, CuI, dimethylformide, 160 ℃; (II) 98% H2SO4, 60 ℃; (III) 1-bromo-2-ethylhexane, K2CO3, dimethylformide, 90 ℃; (IV) Fe, acetic acid, 120 ℃; (V) acetic acid, 120 ℃; (VI) 2-ethyl-1-hexylamine, toluene, 120 ℃; (VII) BF3?Et2O, Et3N, toluene, 120 ℃

Figure 3. Molecular configuration (a, b) and LUMO/HOMO electron cloud distribution (c, d) of 4BN2I calculated by DFT theory

Figure 4. Cyclic voltammetry curves of 4BN2I in solution

	E_1/2^red1 ^a/V	E_pa^ox ^a/V	E_LUMO ^b/eV	E_HOMO ^b/eV	λ_abs ^c/nm	ε_max ^c/ (L•mol⁻¹•cm⁻¹)	E_g^opt ^d/eV	λ_abs ^e/nm
4BN2I	－0.03	＋1.59	－4.77	－6.39	770/705	6.74×10⁴	1.33	706/768

Table 1. Photoelectric properties of 4BN2I

Figure 5. UV-visible absorption spectra of compound 7 and 4BN2I in solution (a) and thin film (b)

Figure 6. Ultraviolet-visible absorption spectra (a) and electron paramagnetic resonance spectra (b) of pristine P3HT film and 4BN2I/P3HT blend film

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具有超低LUMO/HOMO能级的稠环芳烃分子^★